HR: 10:50h
AN: T31H-03    [PDF]
TI: Coexistence of Highly-Depleted Wet Basalts and Depleted Dry Basalts in Sumisu Caldera, Izu-Bonin Arc, Japan
AU: * Tamura, Y
EM: tamuray@jamstec.go.jp
AF: IFREE, JAMSTEC, Yokosuka, 237-0061 Japan
AU: Tani, K
EM:
AF: IFREE, JAMSTEC, Yokosuka, 237-0061 Japan
AU: Chang, Q
EM:
AF: IFREE, JAMSTEC, Yokosuka, 237-0061 Japan
AU: Shukuno, H
EM:
AF: IFREE, JAMSTEC, Yokosuka, 237-0061 Japan
AU: Fiske, R S
EM:
AF: National Museum of Natural History, Smithsonian Institution, Washington, D.C., 20560-0119 United States
AU: Naka, J
EM:
AF: IFREE, JAMSTEC, Yokosuka, 237-0061 Japan
AB: The northern Izu-Bonin arc consists of 11 Quaternary volcanoes and eight Quaternary submarine caldera volcanoes. Sumisu caldera ($31.5\deg$ N, $140\deg$ E), which is 8 $\times$ 9 km in diameter and has 600$\sim$700 m high inner walls, is one of the best developed submarine calderas along the Izu-Bonin arc and has been studied and sampled during the R/V Natsushima NT02-10 and the R/V Kairei KR02-16 cruises in Sept.-October and December, 2002, respectively. The ROV Dolphin 3K, the manned submersible Shinkai 2000 and dredge hauls were used for sea-floor sampling. Both basalt-basaltic andesite ($<$55 wt % SiO$_{2}$) and dacite-rhyolite (66-74 wt % SiO$_{2}$) are clearly predominant eruptive products, but rocks having SiO$_{2}$ contents of 61$\sim$66 wt % are absent. Basalts (49$\sim$53 wt % SiO$_{2}$) from Sumisu caldera and Sumisu island contain 4$\sim$8.5 wt % MgO; variations of major and trace element compositions are relatively large in the basalt-basalt intervals. They ubiquitously bear plagioclase phenocrysts and some contain $>$4 % olivine and augite, but others are free of olivine and/or augite phenocrysts. Many parent-daughter sets within the basalts were examined by least squares mass-balance calculations using phenocryst phases. It is suggested that fractionation alone of phenocryst phases from the most magnesian basalt (8.5 % MgO) cannot explain even the major element variations of the daughter basalts. These discrepancies, however, disappear if the parent basalts were to assimilate small amounts of rhyolite, together with fractional crystallization (AFC). Unfortunately, however, incompatible trace element concentrations, such as Zr and Ba, are not always compatible with these AFC models. The most-magnesian basalt, as well as some of the other basalts, contain low Zr (20$\sim$30 ppm), which cannot yield basalts containing much higher Zr (30$\sim$40 ppm) through fractionation and/or assimilation. Moreover, low-Zr basalts have more light-REE depleted patterns than high-Zr basalts and olivines in low-Zr basalts are more magnesian at a given NiO content than those in high-Zr basalts, suggesting higher degrees of melting of the source mantle. On the other hand, we recognised that high- and low-Zr basalts have different mineral assemblages; low-Zr basalts contain up to 5 vol % augite phenocrysts, but most high-Zr basalts are free of augite phenocrysts. Thus, the former and the latter assemblages are OL + CPX + PL and OL + PL, respectively. H$_{2}$O will retard crystallization of plagioclase. Hydrous basalts will crystallize olivine followed by augite and plagioclase, producing the former assemblage, but plagioclase will appear on the liquidus just after olivine in the dry basalts, yielding the latter assemblage without augite. Moreover, low-Zr basalts are enriched in the fluid mobile element Ba, suggesting a larger fluid content was necessary to produce greater degrees of partial melting. We suggest that there existed dry and wet primary basalts in the Sumisu magmatic system, each having different trace element concentrations and mineral assemblages, which would have been caused by differences of water content in the source mantle and basaltic melt. The lower content of Zr in the wet basalt could then have resulted from higher degree of partial melting of a hydrous source mantle.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1065 Trace elements (3670)
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Tectonophysics [T]
MN: 2003 Fall Meeting